1. Field of the invention
This invention relates to a bismuth/tin alloy. More especially this invention relates to a dental alloy comprising bismuth and tin as base elements and silver and/or antimony as adjuvant elements. This invention particularly contemplates a dental alloy comprising 30 to 74 weight percent bismuth, 19 to 69.9 weight percent tin and either:
A. Silver in an amount of 0.1 to 5 weight percent; or PA0 B. Antimony in an amount of 0.1 to 7 weight percent; or PA0 C. Silver and antimony in a combined amount of 0.1 to 7 weight percent, the proportion of silver being not more than about 5% by weight. PA0 A. Silver in an amount of 0.1 to 5 weight percent; or PA0 B. Antimony in an amount of 0.1 to 7 weight percent; or PA0 C. Silver and antimony in a combined amount of 0.1 to 7 weight percent, the proportion of silver being not more than about 5 percent by weight. PA0 (a) approximately 0.1 to 2.5 weight percent antimony or PA0 (b) approximately 0.1 to 2.0 weight percent silver or PA0 (c) approximately 0.2 to 4.5 weight percent antimony and silver together. PA0 (a) 1.0 to 2.5 weight percent antimony or PA0 (b) 0.5 to 1.0 weight percent silver or PA0 (c) approximately 2.0 to 3.0 weight percent antimony and silver and, if required, also contains: PA0 (d) 0.5 weight percent copper.
2. Discussion of Prior Art
The first step in the manufacture of dentures is for the dentist to take a case of the patient's jaw or an individual tooth, using suitable impression materials. The following are examples of suitable impression materials: elastomers, polysulfide elastomers (thiocols), silicones, hydrocolloids, alginates, plaster and the like.
The dental technician prepares a positive model from the negative mold of patient's jaw or tooth. For this purpose, he uses various model materials, which may expand or shrink. Since the impression materials are also subject to changes in volume, it is important to process the material so as to reduce the tolerance limit to a minimum. This reduces the possible combinations of impression materials and model materials.
The following model materials are available: plaster, plastics, cement, low-melting alloys, amalgams, electroplated metals and metal die-casting alloys. Out of these substances, plaster, plastics and cement are subject to relatively large changes in volume. In addition, plaster softens when heated over 100.degree. C. and is unsuitable as a model material if the polymerization temperatures used are relatively high, e.g., in the manufacture of bridges. There is also a risk of damage through scratching, wear or fracture.
Low-melting casting alloys used as model materials contain, e.g., lead, tin, bismuth, and cadmium and melt at approximately 70.degree. C. These materials are unsuitable in combination with any thermoplastic impression substances. They also have high surface tension, with the result that fine details cannot be very accurately reproduced. Finally, lead and cadmium are poisonous and therefore cannot be used as components of dental alloys.
Copper and silver amalgams are difficult to process when used as model materials, since they take 10 to 12 hours to harden and may release poisonous mercury vapors during processing. Consequently, they are not often used today.
Electroplated metals are applied by electroplating thermoplastic or elastomeric impression materials. The electroplated metal is usually cooper, and the remaining cavities are backed up or lined with plastics or plaster. The dimensional accuracy of this method varies, since the casting impression swells and changes in the electroplating bath. The time taken to manufacture a model--6 to 24 hours--is also excessive.
It is also known to make models from metal die-casting alloys. These are conventional industrial bismuth-tin alloys melting at 138.degree.-170.degree. C., depending on their composition. The dimensional stability can be controlled via the bismuth content, since pure bismuth expands on solidifying. Difficulties occur, however, when spraying narrow spaces, e.g., with regard to surface quality when preparing lower front tooth stumps. In addition, the casting materials are not satisfactoryly wetted by metal in all cases. There are also problems regarding treatment with the lining material. The alloys are somewhat brittle, so that pieces of metal may split off during treatment.
It is an object of this invention, therefore, to improve the known bismuth/tin based die-casting alloys so that they are particularly suitable for use in dental technology. It is a further object to increase the hardness of such alloys without simultaneously making them brittle. Another object is to improve the surface, which is particularly desirable for dental use. A further object is to provide an alloy which is easily removed from the impression materials. When the alloys are sprayed on in layers, the individual layers must adhere firmly to one another and also the lining material.